docs: fix fault-proof proposer.md

[fakedev9999]

Improves fault-proof proposer doc by:

1. fixing typo of the env file name
2. removing unnecessary direction for sourcing env file

[github-actions]

Metric	Value
Batch Start	9,522,963
Batch End	9,522,968
Witness Generation (seconds)	0
Execution Duration (seconds)	21
Total Instruction Count	429,631,246
Oracle Verify Cycles	47,277,822
Derivation Cycles	305,774,228
Block Execution Cycles	7,374,840
Blob Verification Cycles	23,288,882
Total SP1 Gas	666,841,359
Number of Blocks	5
Number of Transactions	5
Ethereum Gas Used	236,128
Cycles per Block	85,926,249
Cycles per Transaction	85,926,249
Transactions per Block	1
Gas Used per Block	47,225
Gas Used per Transaction	47,225
BN Pair Cycles	0
BN Add Cycles	0
BN Mul Cycles	0
KZG Eval Cycles	0
EC Recover Cycles	0
P256 Verify Cycles	0

[Farhad-Shabani]

Should we update challenger.md? That points to the root directory.
Actually, there are other places (search for .env.proposer), we direct readers to the root directory.