Fix doc typo (pandas-dev#29312)

Author: lodagro

pandas/core/computation/eval.py

@@ -119,7 +119,7 @@ def _convert_expression(expr):
     """
     Convert an object to an expression.
 
-    Thus function converts an object to an expression (a unicode string) and
+    This function converts an object to an expression (a unicode string) and
     checks to make sure it isn't empty after conversion. This is used to
     convert operators to their string representation for recursive calls to
     :func:`~pandas.eval`.