Prefix space in additional_options instead of suffix

[shenlebantongying]

Every shell commands in this page is wrong for very long time.

https://ziglang.org/learn/build-system/

The command should be zig build run instead of
.

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After this change:

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The one in ziglang/zig also has this problem btw

https://github.com/ziglang/zig/blob/43e52ec5c5a2267791cb2aaaf37f3f84dbe29d25/tools/doctest.zig#L162

[shenlebantongying]

Same PRs

• shell_out: add whitespace after initial build cmd #1
• Add missing space for build examples #3
• Prefix space in additional_options instead of suffix #4

And also update this line https://github.com/ziglang/www.ziglang.org/blob/1433f40d68ad733819e2ac4ed1132aac663e47d5/build.zig.zon#L6-L9