Fix link syntax for README.md

Github uses different syntax to render README.md and its wiki pages, for
no obvious reason.

Change-Id: I8905bf6ad7a72e6e1bfb3beb83f522ca5956128b
Reviewed-on: http://gerrit.cloudera.org:8080/272
Reviewed-by: Alex Leblang <[email protected]>
Tested-by: Henry Robinson <[email protected]>

README.md

@@ -1,26 +1,26 @@
 # Welcome to Impala
 
-Lightning-fast, distributed [[SQL|http://en.wikipedia.org/wiki/SQL]] queries for petabytes
+Lightning-fast, distributed [SQL](http://en.wikipedia.org/wiki/SQL) queries for petabytes
 of data stored in Apache Hadoop clusters.
 
 Impala is a modern, massively-distributed, massively-parallel, C++ query engine that lets
 you analyze, transform and combine data from a variety of data sources:
 
 * Best of breed performance and scalability.
-* Support for data stored in [[HDFS|https://hadoop.apache.org/]],
-  [[Apache HBase|http://hbase.apache.org/]] and [[Amazon S3|http://aws.amazon.com/s3/]].
+* Support for data stored in [HDFS](https://hadoop.apache.org/),
+  [Apache HBase](http://hbase.apache.org/) and [Amazon S3](http://aws.amazon.com/s3/).
 * Wide analytic SQL support, including window functions and subqueries.
-* On-the-fly code generation using [[LLVM|http://llvm.org/]] to generate CPU-efficient
+* On-the-fly code generation using [LLVM](http://llvm.org/) to generate CPU-efficient
   code tailored specifically to each individual query.
 * Support for the most commonly-used Hadoop file formats, including the
-  [[Apache Parquet|https://parquet.incubator.apache.org/]] (incubating) project.
+  [Apache Parquet](https://parquet.incubator.apache.org/) (incubating) project.
 * Apache-licensed, 100% open source.
 
 ## More about Impala
 
 To learn more about Impala as a business user, or to try Impala live or in a VM, please
-visit the [[Impala homepage|http://impala.io/index.html]].
+visit the [Impala homepage](http://impala.io/index.html).
 
 If you are interested in contributing to Impala as a developer, or learning more about
 Impala's internals and architecture, visit the
-[[Impala wiki|https://github.com/cloudera/Impala/wiki]].
+[Impala wiki](https://github.com/cloudera/Impala/wiki).