Fix typo, 'expontential' --> 'exponential' in fates c starvation model…

[r-ward]

… namelist settings

Description of changes

Fixes a typo in src/utils/clmfates_interfaceMod.F90 for the fates c starvation namelist option ("expontential" --> "exponential") that causes runs using fates_cstarvation_model=exponential (valid options =linear, exponential) to abort at init with error message "Invalid carbon starvation model".

Specific notes

Contributors other than yourself, if any:

• None

CTSM issues resolved or otherwise addressed, if any:

• None

Any user interface changes (namelist or namelist defaults changes)?
None, this makes the already valid exponential fates c starvation model functional

Testing planned or performed, if any:

• tested with single point FATES run at BCI, Panama on Derecho; fix allows running with fates_cstarvation_model=exponential on ctsm5.3.077, PR branch is on current master because change is a single character typo fix, outcome should be identical

Requirements before merge:

• I have followed the CTSM contribution guidelines.

• The code in this PR branch builds with no errors. (one-character string-literal change; identical fix built and run on Derecho; ctsm5.3.077)
• The code in this PR branch runs with no errors. Briefly describe tested configuration(s): tested on ctsm5.3.077
• This either (a) does not change answers, (b) it only changes answers at roundoff level, or (c) I have performed a scientific evaluation of the answer changes. Which?: (a) does not change answers, allows fates_cstarvation_model=exponential configuration to function

• I have reviewed relevant parts of the CLM documentation Tech Note or User's Guide to determine if anything needs to be changed or added. If it does, describe:
• This PR either (a) does not create a need to update the documentation or (b) includes required documentation updates (see guidelines for contributing documentation). Which?:(a)