ref-manual: Grammar correction

I left out the word "not" in a key sentence in the new
"Checking on the Upgrade Status of a Recipe" section.

Little words can make big differences.

(From yocto-docs rev: 2f3995a07eba7e3e3b66aa60bfffcdf9c50077b8)

Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/ref-devtool-reference.xml

@@ -442,7 +442,7 @@
             The command displays a table of your current recipe versions,
             the latest upstream versions, the email address of the recipe's
             maintainer, and any additional information such as commit hash
-            strings and reasons you might be able to upgrade a particular
+            strings and reasons you might not be able to upgrade a particular
             recipe.
             <note><title>NOTES:</title>
                 <itemizedlist>