Fix reference to undefined var `logger` in "Getting Started" example

[mosbasik]

Summary

Just changing the name of one variable in one docs example.

Motivation

While following along with the "Getting Started" guide for the first time, I got sidetracked for several minutes by this logger.

None of the example snippets earlier in the file assigned to a variable named logger (but they frequently assigned the result of structlog.get_logger() to a variable named log).

I spent a bit poking around in the API reference to try and figure out which types define a .bind() method and whether they completely overlap with the types returned by structlog.get_logger().

Eventually I decided the specific variable name probably wasn't a super considered choice and I was overthinking things.

Maybe this change could save someone time in the future.

Further Discussion

It'd be nice (maybe not super valuable, but nice) if a single naming convention were used throughout the docs. I was considering making this a "unify all the names" PR, but figured that might be a waste of time without getting style input from @hynek first.

I found ~71 uses of log and ~41 uses of logger in markdown files (approximately; used a rough regex to try and find relevant cases). Is there a considered reason to use one or the other, or has it been pretty arbitrary?

Some theories I had, but found counterexamples for:

1. maybe you're trying to avoid assigning to a variable the result of evaluating an expression containing that variable
2. maybe one of log/logger is for the result of structlog.get_logger() and the other is for the result of a .bind()

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