Remove invalid syntax in docstrings -S --preview test (#3205)

uR is not a legal string prefix, so this test breaks (AssertionError:
cannot use --safe with this file; failed to parse source file AST:
invalid syntax) if changed to one in which the file is changed. I've
changed the last test to have u alone, and added an R to the test above
instead.

tests/data/miscellaneous/docstring_preview_no_string_normalization.py

@@ -3,8 +3,8 @@ def do_not_touch_this_prefix():
 
 
 def do_not_touch_this_prefix2():
-    F'There was a bug where docstring prefixes would be normalized even with -S.'
+    FR'There was a bug where docstring prefixes would be normalized even with -S.'
 
 
 def do_not_touch_this_prefix3():
-    uR'''There was a bug where docstring prefixes would be normalized even with -S.'''
+    u'''There was a bug where docstring prefixes would be normalized even with -S.'''