docs: change "." to "source"

Change-Id: Ib4e3d1d42de7149e5a64e5f706fa73a80101576a

docs/getting_started.md

diff --git a/docs/getting_started.md b/docs/getting_started.md
index 26ffa2a..c09394f 100644
--- a/docs/getting_started.md
+++ b/docs/getting_started.md
@@ -29,7 +29,7 @@
 $ git clone https://pigweed.googlesource.com/pigweed/pigweed
 ...
 $ cd pigweed
-$ . ./bootstrap.sh
+$ source ./bootstrap.sh
 ...
 ```
 
@@ -131,7 +131,7 @@
 ```bash
 $ git clone https://pigweed.googlesource.com/pigweed/pigweed ~/pigweed
 $ cd ~/pigweed
-$ . ./bootstrap.sh
+$ source ./bootstrap.sh
 ```
 
 **Windows**