[minor] fix grammar error (spring-projects#4013)

add space: that"collects" -> that "collects"

docs/manual/src/docs/asciidoc/index.adoc

@@ -2225,7 +2225,7 @@ The `AuthenticationEntryPoint` is responsible for step three in the above list.
 
 
 ==== Authentication Mechanism
-Once your browser submits your authentication credentials (either as an HTTP form post or HTTP header) there needs to be something on the server that"collects" these authentication details. By now we're at step six in the above list. In Spring Security we have a special name for the function of collecting authentication details from a user agent (usually a web browser), referring to it as the "authentication mechanism". Examples are form-base login and Basic authentication. Once the authentication details have been collected from the user agent, an `Authentication` "request" object is built and then presented to the `AuthenticationManager`.
+Once your browser submits your authentication credentials (either as an HTTP form post or HTTP header) there needs to be something on the server that "collects" these authentication details. By now we're at step six in the above list. In Spring Security we have a special name for the function of collecting authentication details from a user agent (usually a web browser), referring to it as the "authentication mechanism". Examples are form-base login and Basic authentication. Once the authentication details have been collected from the user agent, an `Authentication` "request" object is built and then presented to the `AuthenticationManager`.
 
 After the authentication mechanism receives back the fully-populated `Authentication` object, it will deem the request valid, put the `Authentication` into the `SecurityContextHolder`, and cause the original request to be retried (step seven above). If, on the other hand, the `AuthenticationManager` rejected the request, the authentication mechanism will ask the user agent to retry (step two above).