fix new flake8 error for poor variable name

DocBuild.py

@@ -433,21 +433,21 @@ class DocBuild(object):
         return 0
 
     def MakeYml(self):
-        f = open(self.YmlFilePathBase, "r")
-        f2 = open(self.YmlFilePathOut, 'w')
-        for l in f:
-            f2.write(l)
+        ymlbase = open(self.YmlFilePathBase, "r")
+        yamlout = open(self.YmlFilePathOut, 'w')
+        for line in ymlbase:
+            yamlout.write(line)
 
         # now parse as yaml
-        f.seek(0)
+        ymlbase.seek(0)
         # yaml.load(f)
         # IMPORTANT NOTE: This call to "unsafe_load()" is only acceptable because we control the yml file being loaded.
         #                   Unsafe load is required to support some configuration options for pymdownx.
-        if "extra" in yaml.unsafe_load(f):
+        if "extra" in yaml.unsafe_load(ymlbase):
             raise Exception(
                 "extra: member not allowed in mkdocs_base.yml.  Please add the contents to DocBuild constructor instead.  ")
-        f.close()
-        self.Yml = f2
+        ymlbase.close()
+        self.Yml = yamlout
 
     def CloseYml(self):
         if self.Yml is not None: