Move path to depot to the front of PATH in the tutorial.

Adding it to the end caused issues since a cpplint.py version comes
preinstalled on the system at /usr/bin/cpplint.py, that is actually a symlink to
a bash script.
When the presubmit script is run, it uses python to run this bash script, causing it to fail.

Change-Id: Ib12cbdf20265c14e7252f30c268b5b4a9ce32193
Reviewed-on: https://chromium-review.googlesource.com/999595
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>

man/man7/depot_tools_tutorial.7

@@ -222,13 +222,13 @@ Clone the \fIdepot_tools\fR repository:
 .\}
 .sp
 .sp
-Add \fIdepot_tools\fR to the \fIend\fR of your PATH (you will probably want to put this in your ~/\&.bashrc or ~/\&.zshrc)\&. Assuming you cloned \fIdepot_tools\fR to /path/to/depot_tools:
+Add \fIdepot_tools\fR to the \fIfront\fR of your PATH (you will probably want to put this in your ~/\&.bashrc or ~/\&.zshrc)\&. Assuming you cloned \fIdepot_tools\fR to /path/to/depot_tools:
 .sp
 .if n \{\
 .RS 4
 .\}
 .nf
-\fB$ export PATH=$PATH:/path/to/depot_tools\fR
+\fB$ export PATH=/path/to/depot_tools:$PATH\fR
 .fi
 .if n \{\
 .RE