Fix formatting, capitalization, and grammar in activation issue template (flutter#183061)

Improves documentation quality by:

- Fixing GitHub capitalization
- Improving grammar and wording
- Formatting Pastebin link using markdown
- Using `.txt` file extension format

No functional changes.

Author: sanketudaypatil

.github/ISSUE_TEMPLATE/01_activation.yml

@@ -48,9 +48,9 @@ body:
         please include the output of running them with `--verbose`; for example,
         the output of running `flutter --verbose create foo`.
 
-        If the logs are too large to be uploaded to Github, you may upload
-        them as a `txt` file or use online tools like https://pastebin.com to
-        share it.
+        If the logs are too large to be uploaded to GitHub, you may upload
+        them as a `.txt` file or use online tools like [Pastebin](https://pastebin.com)
+        to share them.
 
         Note: Please do not upload screenshots of text. Instead, use code blocks
         or the above mentioned ways to upload logs.