content/static/doc/tutorial: don't "go run" individual files

Go packages are implicitly made of all the files in a directory,
filtered by build tags. This is a core mechanic of using Go and of
contributing to Go projects. It's also different from many other
languages, including C and Rust, where code in different compilation
units can coexist in the same directory.

The only exception is when Go files are specified on the command line,
in which case a synthetic command-line-arguments package is composed
from them, ignoring build tags.

I don't think it helps beginners to expose them to this exception which
they would otherwise probably never make any use of.

This CL was motivated by hearing of a beginner being confused trying to
contribute to an existing project, because they were trying to "go run"
an individual file, which was failing with a wall of error because of
all the missing symbols from the other files in the package.

Change-Id: Iec0ef73df1c5ada2299d059dd468dc2095c367a8
Reviewed-on: https://go-review.googlesource.com/c/website/+/283635
Trust: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Steve Traut <straut@google.com>

content/static/doc/tutorial/getting-started.html

@@ -114,7 +114,7 @@ <h2 id="code">Write some code</h2>
     <ul>
       <li>
         Declare a <code>main</code> package (a package is a way to group
-        functions).
+        functions, and it's made up of all the files in the same directory).
       </li>
       <li>
         Import the popular
@@ -127,7 +127,7 @@ <h2 id="code">Write some code</h2>
       <li>
         Implement a <code>main</code> function to print a message to the
         console. A <code>main</code> function executes by default when you run
-        code in the file.
+        the <code>main</code> package.
       </li>
     </ul>
   </li>
@@ -136,7 +136,7 @@ <h2 id="code">Write some code</h2>
     Run your code to see the greeting.
 
     <pre>
-$ go run hello.go
+$ go run .
 Hello, World!
 </pre
     >
@@ -252,7 +252,7 @@ <h2 id="call">Call code in an external package</h2>
     Run your code to see the message generated by the function you're calling.
 
     <pre>
-$ go run hello.go
+$ go run .
 go: finding module for package rsc.io/quote
 go: found rsc.io/quote in rsc.io/quote v1.5.2
 Don't communicate by sharing memory, share memory by communicating.