Remove the needless whitespace

Co-authored-by: Lukas Wirth <[email protected]>

src/syntax-extensions/expansion.md

@@ -106,7 +106,7 @@ as many as is needed to completely expand all invocations.
 
 Well, not *quite*.
 In fact, the compiler imposes an upper limit on the number of such recursive passes it is willing to run before giving up.
-This is known as the syntax extension  recursion limit and defaults to 128.
+This is known as the syntax extension recursion limit and defaults to 128.
 If the 128nd expansion contains a syntax extension invocation, the compiler will abort with an error indicating that the recursion limit was exceeded.
 
 This limit can be raised using the `#![recursion_limit="…"]` [attribute][recursion_limit], though it *must* be done crate-wide.