Fix gen_colors docstring: function yields, not returns a list

The gen_colors function is a generator (uses yield) with return type
Iterator[ColorSpan], but its docstring incorrectly stated "Returns a
list of index spans". Updated to "Yield index spans" to accurately
reflect the function's behavior.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Anandesh-Sharma

Lib/_pyrepl/utils.py

@@ -97,7 +97,7 @@ def unbracket(s: str, including_content: bool = False) -> str:
 
 
 def gen_colors(buffer: str) -> Iterator[ColorSpan]:
-    """Returns a list of index spans to color using the given color tag.
+    """Yield index spans to color using the given color tag.
 
     The input `buffer` should be a valid start of a Python code block, i.e.
     it cannot be a block starting in the middle of a multiline string.