Documentation: Various corrections

Correct typos, spelling mistakes, and grammatical errors.

Change-Id: I80ec66de7b2228f9ff45a2f06faf576707195758
diff --git a/Documentation/error-not-a-gerrit-project.txt b/Documentation/error-not-a-gerrit-project.txt
index 368a102..dac98ae 100644
--- a/Documentation/error-not-a-gerrit-project.txt
+++ b/Documentation/error-not-a-gerrit-project.txt
@@ -18,7 +18,7 @@
   project is listed. If the project is not listed the project either
   does not exist or you don't have
   link:access-control.html#category_read['Read'] access for it. This
-  means if you certain that the project name is right you should
+  means if you are certain that the project name is right you should
   contact the Gerrit Administrator or project owner to request access
   to the project.