Fix apt-get command to upgrade bazel

`apt-get upgrade` will upgrade all packages to their latest versions; to upgrade bazel alone, use `install`.

Closes #6483.

PiperOrigin-RevId: 220130623
diff --git a/site/docs/install-ubuntu.md b/site/docs/install-ubuntu.md
index 214246f..c037f04 100644
--- a/site/docs/install-ubuntu.md
+++ b/site/docs/install-ubuntu.md
@@ -103,5 +103,5 @@
 Once installed, you can upgrade to a newer version of Bazel with the following command:
 
 ```bash
-sudo apt-get upgrade bazel
+sudo apt-get install --only-upgrade bazel
 ```