Disable pull_request_target for the vercel action (#969)

`pull_request` and `pull_request_target` are similar and two CI jobs are
triggered.

This is not what we want. This PR disables the `pull_request_target`
event.

Author: seisman

.github/workflows/continuous-documentation.yml

@@ -10,13 +10,6 @@ on:
     - 'pygmt/**'
     - 'README.rst'
     - '.github/workflows/continuous-documentation.yml'
-  pull_request_target:
-    paths:
-    - 'doc/**'
-    - 'examples/**'
-    - 'pygmt/**'
-    - 'README.rst'
-    - '.github/workflows/continuous-documentation.yml'
 
 jobs:
   vercel: