indentation: Fix error message for check-multi-line-strings

A space character managed to escape while developing commit f63e56f.

Author: adam-huganir

yamllint/rules/indentation.py

@@ -302,7 +302,7 @@ def detect_indent(base_indent):
 
         if indent != expected_indent:
             yield LintProblem(line_no, indent + 1,
-                              f'wrong indentation: expected {expected_indent}'
+                              f'wrong indentation: expected {expected_indent} '
                               f'but found {indent}')