Add a missing space in the documentation.

The linked word was attached to the previous text, making it harder to read.
This change adds the missing space needed for it to read more naturally. The
space was missing where doc strings are concatenated.

PiperOrigin-RevId: 611192629
Change-Id: I018e049de972ce2a8113106c089957760981b860

src/main/java/com/google/devtools/build/lib/packages/RuleFunction.java

@@ -26,7 +26,7 @@
         "A callable value representing the type of a native or Starlark rule. Calling the value"
             + " during evaluation of a package's BUILD file creates an instance of the rule and"
             + " adds it to the package's target set. For more information, visit this page about"
-            + "<a href ='https://bazel.build/extending/rules'>Rules</a>.")
+            + " <a href ='https://bazel.build/extending/rules'>Rules</a>.")
 public interface RuleFunction extends StarlarkCallable {
   RuleClass getRuleClass();
 }