Fix/misc docs and tests (#19652)

## Summary
This PR makes a few small consistency fixes:

- Fix outdated GitHub links that still point to the old
`tailwindcss/tailwindcss` org and update them to
`tailwindlabs/tailwindcss` (README + contributing docs + PR template).
- Fix punctuation in the contributing guide (“i.e., …”).
- Update two test titles to use “cannot” instead of “can not” for
consistency.

No behavior changes.

## Test plan
- Link check (manual): clicked the updated GitHub links in `README.md`,
`.github/CONTRIBUTING.md`, and `.github/PULL_REQUEST_TEMPLATE.md` to
confirm they resolve correctly.
- (Optional) `pnpm test` — not required since changes are
docs/test-title-only.

---------

Co-authored-by: Robin Malfait <malfait.robin@gmail.com>

.github/CONTRIBUTING.md

@@ -76,7 +76,7 @@ To run the integration tests, use:
 pnpm build && pnpm test:integrations
 ```
 
-Additionally, some features require testing in browsers (i.e to ensure CSS variable resolution works as expected). These can be run via:
+Additionally, some features require testing in browsers (i.e. to ensure CSS variable resolution works as expected). These can be run via:
 
 ```sh
 pnpm build && pnpm test:ui

packages/tailwindcss/src/css-functions.test.ts

@@ -1361,7 +1361,7 @@ test('resolves paths ending with a 1', async () => {
   `)
 })
 
-test('upgrades to a full JS compat theme lookup if a value can not be mapped to a CSS variable', async () => {
+test('upgrades to a full JS compat theme lookup if a value cannot be mapped to a CSS variable', async () => {
   expect(
     await compileCss(
       css`

packages/tailwindcss/src/index.test.ts

@@ -3098,7 +3098,7 @@ describe('plugins', () => {
     ).rejects.toThrowErrorMatchingInlineSnapshot(`[Error: \`@plugin\` must have a path.]`)
   })
 
-  test('@plugin can not have an empty path', () => {
+  test('@plugin cannot have an empty path', () => {
     return expect(
       compile(
         css`