Update how-to-use-rule-based-ui-context-for-visual-studio-extensions.md

Fix naming mismatch "guidTestUIContext" => "UIContextGuid"

Author: Lennart Brüggemann

docs/extensibility/how-to-use-rule-based-ui-context-for-visual-studio-extensions.md

@@ -64,14 +64,14 @@ Rule-based UI Context can be used in various ways:
 
    ```xml
    <VisibilityConstraints>
-       <VisibilityItem guid="guidTestPackageCmdSet" id="TestId"  context="guidTestUIContext"/>
+       <VisibilityItem guid="guidTestPackageCmdSet" id="TestId"  context="UIContextGuid"/>
    </VisibilityConstraints>
    ```
 
 4. In the Symbols section, add the definition of the UIContext:
 
    ```xml
-   <GuidSymbol name="guidTestUIContext" value="{8B40D5E2-5626-42AE-99EF-3DD1EFF46E7B}" />
+   <GuidSymbol name="UIContextGuid" value="{8B40D5E2-5626-42AE-99EF-3DD1EFF46E7B}" />
    ```
 
     Now, the context menu commands for *\*.config* files will be visible only when the selected item in the solution explorer is a *.config* file and the package will not be loaded until one of those commands is selected.
@@ -146,4 +146,4 @@ Sometimes, packages cannot use static UI Context rules. For example, suppose you
 
 In order to support such packages, rule-based UI Contexts support a hardcoded expression "*" that indicates all the terms below it will be joined with OR. This allows for the master package to define a known rule-based UI Context and tie its command state to this context. Afterwards any MEF extension targeted for the master package can add its terms for editors it supports without impacting other terms or the master expression.
 
-The constructor <xref:Microsoft.VisualStudio.Shell.ProvideExtensibleUIContextRuleAttribute.%23ctor%2A> documentation shows the syntax for extensible UI Context rules.
+The constructor <xref:Microsoft.VisualStudio.Shell.ProvideExtensibleUIContextRuleAttribute.%23ctor%2A> documentation shows the syntax for extensible UI Context rules.