AdvLoggerPkg: Fixed Document for SMM_CORE library instance (#480)

## Description

Readme.md specified that AdvancedLoggerLib SMM_CORE instance was to use
SMM instance.
Corrected document to point to SmmCore instance.

For each item, place an "x" in between `[` and `]` if true. Example:
`[x]`.
_(you can also check items in the GitHub UI)_

- [ ] Impacts functionality?
- **Functionality** - Does the change ultimately impact how firmware
functions?
- Examples: Add a new library, publish a new PPI, update an algorithm,
...
- [ ] Impacts security?
- **Security** - Does the change have a direct security impact on an
application,
    flow, or firmware?
  - Examples: Crypto algorithm change, buffer overflow fix, parameter
    validation improvement, ...
- [ ] Breaking change?
- **Breaking change** - Will anyone consuming this change experience a
break
    in build or boot behavior?
- Examples: Add a new library class, move a module to a different repo,
call
    a function in a new library class in a pre-existing module, ...
- [ ] Includes tests?
  - **Tests** - Does the change include any explicit test code?
  - Examples: Unit tests, integration tests, robot tests, ...
- [x] Includes documentation?
- **Documentation** - Does the change contain explicit documentation
additions
    outside direct code modifications (and comments)?
- Examples: Update readme file, add feature readme file, link to
documentation
    on an a separate Web page, ...

## How This Was Tested
N/A
Documentation change only.

## Integration Instructions
N/A

AdvLoggerPkg/Docs/ReadMe.md

@@ -93,7 +93,7 @@ The following changes are needed in the .dsc for IA32/X64.
   AdvancedLoggerLib|AdvLoggerPkg/Library/AdvancedLoggerLib/Smm/AdvancedLoggerLib.inf
 
 [LibraryClasses.X64.SMM_CORE]
-  AdvancedLoggerLib|AdvLoggerPkg/Library/AdvancedLoggerLib/Smm/AdvancedLoggerLib.inf
+  AdvancedLoggerLib|AdvLoggerPkg/Library/AdvancedLoggerLib/SmmCore/AdvancedLoggerLib.inf
 
 [LibraryClasses.X64.DXE_RUNTIME_DRIVER]
   AdvancedLoggerLib|AdvLoggerPkg/Library/AdvancedLoggerLib/Runtime/AdvancedLoggerLib.inf