docs: fix example for filtering by directory and scm (#8814)
### Description
Closes #8762 by updating docs to provide the correct filter for the
description.
Multiple filters are combined via union so `--filter=./apps/*
--filter=[HEAD^1]` is really "all packages that are in `apps/` *or* have
changed since `HEAD^1`".
### Testing Instructions
Quick verify that my understanding is correct:
```
# Only web was changed in the last commit
[0 olszewski@chriss-mbp] /tmp/foobar $ turbo build --filter='[HEAD^1]' --dry=json | jq '.tasks | map(.taskId)'
[
"web#build"
]
# All of these are run if we filter to all packages in apps/
[0 olszewski@chriss-mbp] /tmp/foobar $ turbo build --filter='./apps/*' --dry=json | jq '.tasks | map(.taskId)'
[
"@repo/eslint-config#build",
"@repo/typescript-config#build",
"@repo/ui#build",
"docs#build",
"web#build"
]
# Union is taken with multiple filters
[0 olszewski@chriss-mbp] /tmp/foobar $ turbo build --filter='./apps/*' --filter='[HEAD^1]' --dry=json | jq '.tasks | map(.taskId)'
[
"@repo/eslint-config#build",
"@repo/typescript-config#build",
"@repo/ui#build",
"docs#build",
"web#build"
]
# Intersection is taken with this syntax
[0 olszewski@chriss-mbp] /tmp/foobar $ turbo build --filter='{./apps/*}[HEAD^1]' --dry=json | jq '.tasks | map(.taskId)'
[
"web#build"
]
```
